It is well known in the art that the resistance modulation of SMR devices can be employed in position and speed sensors with respect to moving ferromagnetic materials or objects (see for example U.S. Pat. Nos. 4,835,467, 4,926,122, and 4,939,456).
The shortcoming of SMR devices is their temperature sensitivity. They have a negative temperature coefficient of resistance and their resistance can drop as much as 50% when heated to 180 degrees Celsius. Generally, this led to the use of SMR devices in matched pairs for temperature compensation. Additionally, it is preferable to drive SMR devices with current sources since, with the same available power supply, the output signal is nearly doubled in comparison with a constant voltage source.
To compensate for the SMR resistance drop at higher temperatures, and thus, the magnitude decrease of the output signal resulting in decreased sensitivity of the SMR device, it is also desirable to make the current of the current source automatically increase with the SMR temperature increase. This is shown in U.S. Pat. No. 5,404,102 in which an active feedback circuit automatically adjusts the current of the current source in response to temperature variations of the SMR device. It is also known that air gap variations between the SMR device and ferromagnetic materials or objects will affect the resistance of SMR devices with larger air gaps producing less resistance and decreased output signals.
What is needed is a less complicated method and apparatus having the features of a current source and employing an automatically adjustable current to compensate for decreased SMR sensitivity at high temperatures and large air gaps.
A circuit of interest in this regard, well known in the art, is a conventional current mirror circuit 10 (often referred to simply as a current mirror), shown in FIG. 1. In current mirror circuit 10, the reference resistor R has a fixed value and, in conjunction with the constant voltage source V.sub.SS and transistor Q.sub.1, determines the magnitude of the reference current I.sub.R. The electronic operation of the current mirror circuit 10 dictates that the current I.sub.0 will have approximately the same magnitude as the reference current I.sub.R provided that transistors Q.sub.1 and Q.sub.2 are matched. Thus, the reference current I.sub.R is mirrored to be the collector current I.sub.0 of transistor Q.sub.2. In FIG. 1, I.sub.0 is conventionally called the mirror current or mirrored current and the "mirrored portion" of the current mirror circuit 10 will be designated the mirrored circuit 12.
Accordingly, it would be desirable if somehow the current mirroring feature of a mirror circuit could be adapted to provide a current source employing an automatically adjustable current to compensate for decreased SMR device sensitivity at higher temperatures and large air gaps without the need for an active feedback circuit.